The present invention relates to electroluminescent devices, and more particularly, to an improved method for sealing an organic light emitting display to increase the lifetime of the display.
Organic light emitting devices (OLEDs) are emissive displays consisting of a transparent substrate coated with a transparent conducting material, such as Indium Tin oxide (ITO), one or more organic layers, and a cathode made by evaporating or sputtering a metal of low work function characteristics, such as Ca or Mg. The organic layers are chosen so as to provide charge injection and transport from both electrodes into the electroluminescent organic layer (EL) where the charges recombine, emitting light. There may be one or more organic hole transport layers (HTL) between the ITO and the EL, as well as one or more electron injection and transporting layers between the cathode and the EL.
OLEDs hold out the promise of providing inexpensive displays. To function over extended periods of time, an OLED must be sealed to prevent water and oxygen from reaching the cathode and polymer layers. The low work function cathode metals are easily oxidized upon exposure to moisture and oxygen. Once oxidized, the cathode metal can no longer function as a charge injector. Hence, the light output of the OLED drops due to higher resistance and lower electron injection efficiency.
One method for improving the moisture resistance of an OLED involves adding a layer of a more stable metal, such as aluminum, on top of the low work function cathode to provide a moisture barrier. Unfortunately, such barriers do not provide sufficient protection.
A second prior art method for preventing moisture damage utilizes a package with a back cover made of either metal or glass. The back cover is typically glued to the substrate on which the OLED is fabricated using an epoxy adhesive. Unfortunately, epoxy adhesives do not provide sufficient moisture protection. Hence, to further prevent moisture from attacking the cathode metals, a moisture getter is sandwiched between the substrate and back cover. The moisture getter absorbs the moisture that passes through the epoxy adhesive before the moisture can reach the cathode metal. The moisture getter has two problems. First, the moisture getter has a saturation point where it will no longer be effective. Hence, the getter only protects the OLED for some period of time. Second, the absorbed moisture in the getter may be released when the package is heated up, resulting in cathode metal oxidation.
Another method for providing a moisture barrier for use in flexible OLEDs is taught in U.S. Pat. No. 6,146,225. In this method the flexible substrates are coated with a moisture barrier consisting of a repetition of polymer layer sandwiched between inorganic dielectric (nitride or oxide) layers. This approach requires the generation of a very smooth polymer layer on which the dielectric is deposited. If the polymer layer has imperfections, then the inorganic layer will develop pinholes, and the high level isolation observed with layers of this type is substantially reduced.
Broadly, it is the object of this invention to provide an improved sealing system for OLEDs and similar structures.
This and other objects of the invention will become apparent from the following detailed description of the invention and the accompanying drawings.
The present invention is an electroluminescent device and method for making the same. The electroluminescent device includes a substrate that is impermeable to water and oxygen having a first electrode thereon. An electroluminescent layer is in electrical contact with the first electrode, and a second electrode is in electrical contact with the electroluminescent layer. A seal having a layer of epoxy and a layer of silicon nitride prevents water and oxygen from reaching the second electrode. The epoxy preferably has a cure temperature less than 140xc2x0 C. Similarly, the SiNH layer is preferably deposited at a temperature below 140xc2x0 C. The layer of SiNH preferably has an etch rate of less than 50 A/sec in 10% HF solution.